Can't agree more on Mr David Kyle Latinis suggestion. Site-specific GPR imageries can be freely sorted only from the primary researchers. If un-fortunate, you can approach the concerned institution that undertook the project, however, chances are lower they would share it for free.
If you interested in original GPR data to learn data processing and imaging, you will need access to GPR data processing and imaging software. If you would like to interpret GPR data archaeologically, are you interested in profile sections (B-scans) or horizontal time- or depth-slices (C-scans)? In the second case, which area/region/type of sites interests you?